The present invention generally relates to the field of endodontic instruments, and more particularly, to a conical blank and to a method of producing cut blades for instruments used in root canal treatment.
Endodontic instruments of this type are mainly composed of a blade and a shaft. The blade is inserted into the dental cavity to be treated and requires a high degree of flexibility in order to follow the path of the root canal in the tooth. A high degree of mechanical strength is also required in order not to accidentally break the blade during working of the canal. The blade defines an active part having a generally conical shape.
Such blades were initially made of steel. As these blade were used, however, it became apparent that steel was not sufficiently flexible and strong to satisfy the demands placed on the blades, particularly considering the small diameter of such blades.
An article by Walia et al., published in the Journal of Endodontics in July of 1988 (Volume 14, No. 7, pages 346-351), described the production of endodontic instruments from a nickel/titanium alloy. In particular, K-type files were made from a wire of nickel/titanium with a diameter of 0.020 inches (i.e., about 0.508 mm), using a method which started directly from a blank cut in the wire.
Using the very strong nickel/titanium alloys satisfied the basic requirements for endodontic applications, but posed a problem concerning the speed of the machining operation. This, in turn, posed a problem relative to the cost of manufacturing cut blades using the traditional grinding techniques employing cylindrical blanks.
Endodontic instruments, for example, those known as Hedstroem files, have for many years been made by machining, in particular, by grinding. The process begins with a cylindrical blank, which is brought in front of a grinder. The grinder has a configuration such that machining of the blank produces a groove in the blank, i.e., a “cutting edge”, and provides the instrument with a desired conicity by virtue of the combined advancement of the blank in front of the grinder, and of rotation of the blank about its axis. Such instruments are described, for example, in standard ISO 3630.
Methods for producing blades having one or more cutting edges are known, in particular, from U.S. Pat. No. 5,527,205, U.S. Pat. No. 5,628,674 and U.S. Pat. No. 5,655,950, which describe the use of cylindrical blanks formed of alloys of at least 40% titanium and about 50% nickel, and having a diameter of less than 0.07 inches. The blades obtained by the disclosed methods have at least one channel cut in a helical formation, in a single pass (i.e., displacement) in front of the grinder. The disclosed methods are said to obtain blades which are free from defects, and without deformation of the metal. U.S. Pat. No. 5,527,205 and U.S. Pat. No. 5,655,950 additionally specify that at least 25% of the diameter of the cylindrical blank is removed from the portions of the blade which are to perform the greatest amount of cutting.
Such processes for producing conical endodontic instruments from a cylindrical blank were known at the time the above-identified patents were filed. For example, such methods were disclosed by Sylvie Yguel Henry, in a thesis presented in Nancy, France on Jul. 6, 1988, and by Marie-Christine Spohr, in a thesis presented in Nancy, France on Apr. 29, 1987. Such production methods were also discussed by Alain Cavalli, in a thesis presented in Marseilles, France on Mar. 29, 1982.
Producing cut blades from a cylindrical blank, whether formed of steel or formed of a nickel/titanium alloy, entails considerable difficulty in obtaining the overall conical shape of the cut blade. This conicity can vary from one piece to another, and in all cases, portions of the cut blade will have a reduced cross-section, which then requires a substantial removal of material by abrasion.
For portions of the cut blade which have a greater diameter, closer to the diameter of the cylindrical blank, the cutting speed can be more rapid. This, however, results in a differential removal of material along the cut part, which in turn poses various technical problems resulting in a considerable length of time for the production of the machined piece. This then results in a high cost for the production of each endodontic instrument.
Consequently, it is the object of the present invention to improve the speed of production for cut blades usable in the endodontic field, and more particularly, to solve this principal difficulty in association with the production of instruments made of nickel/titanium.